1. Field of the Invention
The present invention provides a method and apparatus for optically imaging the inner wall of a tubular tissue and more particularly, provides an intra-vascular device for the optical imaging of plaque and the like on the inner wall of a blood vessel.
2. Description of the Prior Art
Interventional catheter-based therapeutic techniques have recently become important as they may avoid the necessity of less conservative surgical procedures. Techniques to widen a luminal obstruction in an artery include balloon dilation, tip abrasion, atherectomy, photodynamic therapy and laser ablation.
The primary limitation in choosing the appropriate therapeutic modality for cardiovascular intervention is a lack of information about the vessel being treated. Visualizing the extent of the disease and guiding an interventional device is severely limited by the present imaging technology. The most commonly employed and least informative modality for vascular imaging is angiography. Newer modalities such as angioscopy, spectroscopy, extravascular ultrasound, and intra-vascular ultrasound have shown varying degrees of success.
Angiography provides a two-dimensional blood flow map. A radiopaque dye is injected into the vessel being visualized and x-ray fluoroscopy indicates the path of the dye flow as a dark projection. Angiography does not give information about the vessel but indicates the remaining open lumen. Angiography only shows the border between the blood and the innermost surface of the vessel. If blood flow is completely occluded, angiography is useless. Since this process is a two-dimensional view of the vessel, an occluded vessel may appear completely open in one projection while a 90-degree rotation of the projection would clearly show a severely occluded vessel.
Angioscopy employs a small diameter low profile highly flexible endoscope. Angioscopy only supplies information of the luminal surface. Such topographical information is marginally useful at best. For angioscopy to function, the optical pathway between the imaging device and the vessel wall must be clear. This is achieved by either large volumes of saline flush or by total occlusion of the vessel. Small quantities of saline flush will not clear the vessel adequately for this procedure. Large quantities of saline flush can (a) cause an electrolyte imbalance leading to congestive heart failure or (b) cause ischemia due to a lack of perfusion of oxygen rich blood to muscles. If the arteries being treated are the coronaries, prolonged ischemia in the heart is not always well tolerated. Again, in a total occlusion, angioscopy is useless.
Spectroscopy to differentiate diseased and healthy tissue has had limited success in so-called "smart laser" systems. These devices have employed bundles of fibers to detect atheromatous plaques. The technology has been limited by the small number of fibers employed as discrete detectors. This geometry relays information which is too segmented and often conflicting to be useful in the decision-making process.
Extravascular ultrasound vessel imaging or color flow Doppler imaging has been used for quite some time. The information provided is of a gross nature not generally applicable as a detailed imaging or guidance system. The technology is based on the concept that there is a frequency shift in reflected ultrasound waves which may be used to measure flow velocity. Color flow Doppler is very useful for detecting flow conditions non-invasively, but cannot provide the detailed information necessary for intra-vascular applications.
Intra-vascular ultrasound (IVUS) has gained in popularity in the past few years. IVUS goes beyond angiography and angioscopy by supplying information below the intimal surface of the vessel to create a computer generated cross-sectional view of the vessel which can be displayed on a video monitor. The architecture of the vessel including intima, media, and adventitia along with atheromatous plaques and calcified lesions is well defined and easily observed.
In view of the detailed information from IVUS it is advantageous to develop an analogous optical system. An intravascular optical radial imaging (IVORI) system supplies both topographical and subintimal radial information on the composition of a vessel which can be displayed as a cross-sectional view or as a 3-dimensional reconstruction of the vessel. Such data provides real-time high resolution information about the vessel architecture to: (a) indicate diseased and healthy parts of the vessel, (b) guide an interventional therapy such as Percutaneous Transluminal Coronary Angioplasty (PTCA) or laser angioplasty, (c) guide an interventional therapy such as atherectomy which works best in eccentric lesions (which are not always visible and locatable under fluoroscopy), and (d) help the clinician in the decision-making process.